10 TB HDDs is available at 300-350 dollars at the time being. Wdl ~ 10 TB.

Yes, time too. Games finish earlier with the same adjudication parameters; the bigger the TB set, the shorter the games are.

I was thinking more along the lines of 8TB HDD + 2TB SSD. I already have a Seagate Archive, I would only need to offload the most relevant content to Blu-Ray discs (already cheaper than HDD space) and delete the rest. The Crucial MX500 is also around 300 for the 2TB model, which is a good price for such a large SSD.

I have looked recently at merging the CFish version of tbprobe.c into my fork of Fathom, but there a lot of diffs, so that it is kind of a tall order. And I don't have 7-man bases to test it with. Pull requests are welcome, though.

So WDL without DTZ is perfectly possible. As far as cursed/blessed wins/losses are concerned (so whether the 50-move rule makes a difference), the answers you get will be accurate only if you probe immediately after a capture or pawn move.

When adjudicating 7-piece positions, you will probe only immediately after the capture that brought the game from 8 to 7 pieces. So WDL will give the correct answer.

What is not needed for WDL are the tables that can be reached through promotions.

So if you are only interested in KRPPvKRP and only need WDL, then you must download just KRPPvKRP and all 6-piece WDL TBs (or at least those that can be reached through captures, but you're not going to gain much by being selective with 6-piece tables if you are downloading the much bigger 7-piece tables). You don't need all KRXYvKRZ if you are only interested in adjudicating KRPPvKRP positions or in analysing positions for which KRPPvKRP is important.

(But if you want to play out KRPPvKRP positions to mate reliably, you need all the tables that can be reached through promotion and the corresponding DTZ tables.)